bnxt_en: Set backplane link modes correctly for ethtool

Use the return value from bnxt_get_media() to determine the port and
link modes.  bnxt_get_media() returns the proper BNXT_MEDIA_KR when
the PHY is backplane.  This will correct the ethtool settings for
backplane devices.

Fixes: 5d4e1bf6 ("bnxt_en: extend media types to supported and autoneg modes")
Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Signed-off-by: Shravya KN <shravya.k-n@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>